Modern batteries often include multiple battery cells connected in series, and multiple batteries can be connected in series to form a battery module. Unfortunately, the actual output voltage provided by each individual battery cell in a battery or each battery in a battery module may vary slightly. This can be caused by any number of factors, such as manufacturing variations, temperature variations, or other internal or external factors. This can cause problems during charging and discharging of the battery cells or batteries. In some systems, voltage detection circuitry can be used to determine the output voltage of each battery cell or battery, and a voltage balancing system can be used to compensate for variations in the output voltages.
Consider battery cells connected in series, where each battery cell is designed to provide an output voltage of 3.8V. Voltage detection circuitry may determine that one of the battery cells actually has an output voltage of 3.9V. A conventional passive voltage balancing system typically includes resistors that dissipate electrical energy from battery cells or batteries having excessive output voltage. In this example, the dissipation of electrical energy causes the 3.9V output voltage to drop to the desired level of 3.8V. However, since electrical energy is dissipated using the resistors, this can result in significant energy being lost from the battery cell, which shortens the operational life of the battery.